This invention relates to a hydraulic control circuit for a fluid coupling equipped with a lock-up clutch and finds use in, e.g. a torque converter having a lock-up clutch for an automatic transmission.
An example of a conventional control circuit for a fluid coupling with a lock-up clutch has already been proposed in Japanese Pat. KOKAI Publication No. 60-30864. In order to operate the lock-up clutch in a torque converter efficiently, this conventional control circuit is adapted to lower the pressure inside the torque converter in response to actuation of the lock-up clutch, thereby enlarging the differential pressure between the hydraulic pressure of the lock-up clutch and the hydraulic pressure of the torque converter. The pressure in the torque converter is regulated to a constant pressure with respect to the throttle opening. At the same time, lubrication pressure is also regulated to a constant value. A problem is that since lubrication must be regulated to a pressure capable of assuring a requisite lubricant flow rate at full throttle, the amount of lubricant which flows at low throttle is greater than the optimum amount of lubricant flow and there is an increase in oil pump loss.
Similarly, the pressure in the torque converter at low throttle is higher than the optimum pressure. Therefore, in order for the differential pressure between the hydraulic pressure of the lock-up clutch and the torque converter pressure to be provided with a prescribed magnitude, the lock-up torque hydraulic pressure must be comparatively high. This has an adverse effect upon efficiency of the oil pump.